Directed self-assembly (“DSA”) processes use block copolymers to form lithographic structures, which are formed by the rearrangement of the block copolymer from a random, unordered state to a structured, ordered state. The morphology of the ordered state is variable and depends on a number of factors, including the relative molecular weight ratios of the block polymers, as well as the surrounding chemical and physical environment. Common morphologies include line-space and cylindrical, although other structures may also be used. For example, other ordered morphologies include spherical, lamellar, bicontinuousgyroid, or miktoarm star microdomains.
Two common methods used to guide self-assembly in BCP thin films are grapho-epitaxy and chemo-epitaxy. In the grapho-epitaxy method, self-organization of block copolymers is guided by pre-patterned substrates. Self-aligned lamellar BCPs can form parallel line-space patterns of different domains in topographical trenches and enhance pattern resolution by subdividing the space of topographical patterns. However, defects and line-edge roughness are easily induced in this grapho-epitaxy directed self-assembly scheme. For example, if the sidewalls are neutral, the lamellae tend to orient perpendicular to the sidewalls and will not subdivide the pitch along the desired direction.
In the chemical epitaxy method, the self-assembly of BCP domains is guided by chemical patterns having pitch dimensions commensurate with the domain size or pitch period (L0) of the self-assembled BCP morphology. The affinity between the chemical patterns and at least one of the types of BCP domains results in the precise placement of the different BCP domains on respective corresponding regions of the chemical patterns, i.e., a pinning region. The affinity for the one type of domain (for example the A domains of an A-B diblock copolymer assembly) dominates the interaction of the other domain(s) (for example the B domains) with the non-patterned regions of the surface, which can be selective or non-selective (or neutral) towards the other type(s) of domains. As a result, the pattern formation in the resulting BCP assembly can directly mirror the underlying chemical pattern (i.e., can be a one-for-one reproduction of the features of the chemical pre-pattern). Moreover, depending on the domain size or pitch period (L0) of the self-assembled BCP morphology and the critical dimension (CD) of the pinning regions and the non-patterned regions, frequency multiplication can be achieved. However, dimension control and line-edge roughness can be negatively affected in chemo-epitaxy methods by topographical variations in the chemical pre-pattern.
One commonly observed variation in chemical pre-patterns is introduced when forming a neutral layer over the prepattern, which is not resolved in the subsequent lift-off step. Referring to FIGS. 1A-1E, a layered substrate 100 of the prior art is provided having a substrate 101 coated with an under-layer 102 and an imaged layer of photoresist 103, wherein exposed regions 104 and unexposed regions 105 are shown after exposure to radiation 106. Where the layer of photoresist 103 is a positive tone photoresist comprising a photoacid generator, exposed regions 104 are rendered soluble to positive tone developing chemistry, such as aqueous tetramethylammonium hydroxide (TMAH), upon performing a post-exposure bake. As shown in FIG. 1B, exposure of layer of photoresist 103 to a developing chemistry removes exposed regions 104 to provide openings 107. A flood exposure step shown in FIG. 1C, followed by a bake step provides positive resist lines 108. Coating the positive resist lines 108 with a thin uncross-linked neutral layer, which is subsequently baked to cross-link the thin neutral layer, forms a cross-linked neutral layer 110. However, as shown in FIG. 1D, the application of the neutral layer 110 has traditionally been plagued by pooling regions 110a of the neutral layer 110 against the sides 108a of the positive resist lines 108. These pooling regions can be seen in the scanning electron micrograph (SEM) shown in FIG. 2A.
The underlying positive tone resist lines 108 are then lifted off by exposure to a developer solution (such as a tetramethyl ammonium hydroxide (TMAH) solution), which penetrates the thin cross-linked neutral layer 110, and then dissolves the underlying positive tone resist lines 108. As the positive tone resist lines 108 dissolves, the cross-linked neutral layer 110 attached to the lines 108 is essentially lifted off the layered substrate 100 because it has lost its underlying support, i.e., the positive tone resist lines 108. Unfortunately, the pooling regions 110a of the neutral layer 110 are not removed in this subsequent developing step. Accordingly, this pre-pattern variation 112a remains in the patterned neutral layer 112. In the SEMs shown in FIGS. 2B and 2C, the retained pooling regions are prominently evident in the bulbous edges. The increased variations in neutral layer topography can lower the propensity of an overlying layer of BCP to undergo DSA, which in turn causes an increase in defects.
Therefore, due to the aforementioned limitations, methods to improve neutral layer topography would therefore be highly desirable for improved directed self-assembly processes.